Epsilon caprolactam, usually referred to simply as "caprolactam", is a large volume commodity chemical used as a monomer in the production of the commercially important Nylon-6. Although routes to the precursor cyclohexanone oxime vary, all commercial caprolactam production makes use of a Beckmann rearrangement of the oxime. The commercial reaction is carried out in a batch operation in oleum (H.sub.2 SO.sub.4.SO.sub.3) solution. The recovery step in this technology employs an ammonium hydroxide neutralization of the resulting caprolactam-oleum solution, a process generating two moles of by-product ammonium sulfate per mole of product. The sulfate has some value as a low grade fertilizer, but its recovery and/or disposal can add substantial cost to Nylon-6 production. Attempts have been made to circumvent the use of oleum and carry out the reaction in the gas phase, thereby eliminating the undesirable by-product.
A number of patents and publications have appeared which describe such heterogeneous gas phase conversions. Examples of these include U.S. Pat. No. 3,503,958 to P. S. Landis, which describes and claims such conversion using a zeolite such as hydrogen Y; U.S Pat. No. 3,016,375 which uses as catalyst polyphosphoric acid; and U.S. Pat. No. 4,359,421 to Bell et al. which uses as catalyst a zeolite having a silica to alumina ratio of at least 12 and a Constraint Index of 1 to 12. The foregoing patents are incorporated herein by reference for background purposes.
U.S. Pat. No. 4,582,815 to Bowes describes and claims a method for preparing binder-free and silica-bonded extrudates of zeolites, including ZSM-5. The entire contents thereof are incorporated herein by reference as if fully set forth.
The desired rearrangement of cyclohexanone oxime (I) to caprolactam (II) is believed to occur via a protonated intermediate (not shown), according to Equation A. ##STR1##
In addition, however, two major side reactions occur, one with the formation of 5-cyanopentene (III) and water (Equation B), and the other forming cyclohexanone (IV) (Equation C). ##STR2##
In addition to (III) and (IV) other by-products of unknown structure also are formed.
While the heterogeneous gas phase processes described above for conversion of cyclohexanone oxime to caprolactam all avoid the problems associated with the ammonium bisulfate product formed in the conventional oleum process, these processes in general tend to form an uneconomically large amount of organic by-products. There is evident need for a heterogeneous process which is highly selective for the desired epsilon caprolactam product.
We have now found that an unexpected, large increase in selectivity is obtained when cyclohexanone oxime is converted to epsilon caprolactam using, as catalyst, an intermediate pore-size zeolite having acid activity within a prescribed range. The improved selectivity is accompanied by better (slower) aging of the catalyst.